The invention pertains to a device for measuring pressure or differential pressure, with an oxide ceramic pressure measurement cell, surrounded by a connection housing, for converting the pressure or differential pressure impinging on the pressure measurement cell into an electrical quantity, with an interchangeable connector element penetrated by a centered hole for the physical connection of the measuring space containing measuring medium to the pressure measurement cell and/or the simultaneous attachment of the device at the measurement site.
Devices of this type have the objective of displaying the pressure or differential pressure impinging on the measuring medium, for instance in the inside of containers, conduits and the like, that is, in the measurement space, or of feeding it to a control room for display or to activate agents to intervene and control the course of the processes. For this purpose it is necessary to arrange these devices in such a way at the measurement site that they are mechanically fastened and that the pressure measurement cell to be acted upon by the pressure or differential pressure is in direct physical connection to the measuring space and thus the measuring medium.
If, in accordance with the state of the art commonly applied today, the pressure measurement cell is an oxide ceramic pressure measurement cell, then in order to achieve an exact measurement not influenced by temperature changes, it is necessary to minimize strain on the pressure-sensitive oxide ceramic pressure measurement cell inside the measuring device.
For the physical connection of the pressure measurement cell to the measuring medium, as well as the electrical connection of the sensor to a display unit, U.S. Pat. No. 4,825,710 proposes an electrical pressure sensor which has conical threaded holes in the housing bottom as well as in the electrical connector space lid, into which, depending on the necessary connection, interchangeable connector pieces or nipples are inserted. For the positive connection of housing and connector space lid, a loose clamping ring encloses a collar of the housing. The theory of this patent gives no indication of a minimized strain arrangement of an oxide ceramic transducer.
U.S. Pat. No. 4,617,607 discloses a capacitive high-pressure converter, in which an oxide ceramic pressure measurement cell is surrounded by a housing. A threaded ring is provided for fastening the pressure measurement cell, which is supported on an annular surface with an interposed spacer ring and which presses the transducer on its membrane side against a housing lid. The disadvantage of such an arrangement is that the pressure measurement cell is not supported in a minimized strain fashion and temperature-induced changes of the contact pressure have a deleterious effect on the measurement accuracy. Furthermore, the connector element is permanently connected by means of a welded bond to the housing surrounding the converter, so that removal and exchange of the connector element is not possible.
A capacitive pressure sensor is disclosed in German Patent No. 2,124,979 in which the pressure converter is surrounded by a housing corresponding to the general state of the art. The pressure converter here is connected to the pressure source by a flow channel penetrating a contact element. The contact element has means for fastening the sensor at the measuring site, for instance, in an internally threaded hole in the wall enclosing the measuring medium or in a mounting device, as well as means for introducing a torque. It is essential to the function of the pressure sensor that the flow channel forming the connection between pressure converter and pressure source be absolutely pressure-tight. In order to attain this, this publication proposes connecting the contact body to the housing by insertion, that is, nonremovably, by means of a press fit.
Another pressure pickup and a process for the calibration of pressure pickups can be derived from European Patent (OLS) 410,014. Therein it is proposed to form cross-shaped flexible diaphragms on the level of the measuring membrane by means of cylindrical recesses, which are equipped with elongation sensors. The pressure pickup additionally has a coaxially formed cylindrical recess into which a pressure connection element is inserted. The pressure connection element here likewise represents a connector stem with which the pressure pickup can be attached at the measuring site. A pressure medium channel running through its center also represents here the connection between the measuring medium and the measuring membrane. The pressure connector element also has means for holding the pressure pickup in place as well as means for introducing a torque. To provide absolute pressure-tightness between membrane and connector element it is proposed that both parts be connected nonremovably by a welded joint or some other appropriate joining technique.
Both of the latter proposed devices share the disadvantage in light of the known state of the art that the contact and/or connection means are not variable. Depending on the measuring objective it is necessary to satisfy different requirements for the attachment of the pressure or differential pressure pickup at the measurement site and/or connection with the measuring medium while simultaneously maintaining a minimized uniform strain arrangement of the pressure measurement cell in the interior of the housing. Thus, it is necessary to provide different diameters of connector thread or the flange depending on the pressures to be accommodated and the wall surrounding the measuring medium or the special mounting device. But for aggressive, corrosive and/or explosive media, the design of the attachment and/or connection elements permits only certain suitable materials, which, depending on the aggressiveness, corrosion, explosiveness, temperature and so on, are the only ones which may be allowed to come into contact with the measuring medium. In addition, different regulations and standards must be observed, which likewise lead to different attachment and/or connector elements for different countries. Finally, depending on the application it is necessary to provide the attachment and/or connection elements as threaded stems or as attachment flanges.
In light of the above, the invention sets the objective for itself of specifying a device for measuring pressure or differential pressure that guarantees the necessary minimized uniform strain seating of the oxide ceramic pressure measurement cell, as well as the absolute necessary pressure-tightness of the measuring unit, and additionally forms a releasable connection between measurement pickup and attachment .and connector elements, which removes previous disadvantages and represents a considerable reduction of costs, and the measuring device enclosed by an intermediate housing can remain unchanged independently of the various demands.
An additional advantage of the invention consists in the fact that the membrane of oxide ceramic measurement transducer, is easily accessible, for cleaning purposes, for instance, and that hence the sealing elements can be easily exchanged.
This objective is attained by the characteristic features of claim 1.
Additional advantageous configurations of the invention are characterized in the subclaims.